Vertebrates are constantly threatened by the invasion of microorganisms and have evolved mechanisms of immune defence to eliminate infective pathogens. In mammals, this immune system comprises two branches; innate immunity and acquired immunity. The first line of host defence is the innate immune system, which is mediated by macrophages and dendritic cells. Acquired immunity involves the elimination of pathogens at the late stages of infection and also enables the generation of immunological memory. Acquired immunity is highly specific, due to the vast repertoire of lymphocytes with antigen-specific receptors that have undergone gene rearrangement.
Central to the generation of an effective innate immune response in mammals are mechanisms which bring about the induction of interferons and other cytokines which act upon cells to induce a number of effects. In man, the type I interferons are a family of related proteins encoded by genes on chromosome 9 and encoding at least 13 isoforms of interferon alpha (IFNα) and one isoform of interferon beta (IFNβ). Interferon was first described as a substance which could protect cells from viral infection (Isaacs & Lindemann, J. Virus Interference. Proc. R. Soc. Lon. Ser. B. Biol. Sci. 1957: 147, 258-267). Recombinant IFNα was the first approved biological therapeutic and has become an important therapy in viral infections and in cancer. As well as direct antiviral activity on cells, interferons are known to be potent modulators of the immune response, acting on cells of the immune system (Gonzalez-Navajas J. M. et al Nature Reviews Immunology, 2012; 2, 125-35).
Toll-like receptors (TLRs) are a family of ten Pattern Recognition Receptors described in man (Gay, N. J. et al, Annu. Rev. Biochem., 2007: 46, 141-165). TLRs are expressed predominantly by innate immune cells where their rôle is to monitor the environment for signs of infection and, on activation, mobilise defence mechanisms aimed at the elimination of invading pathogens. The early innate immune-responses triggered by TLRs limit the spread of infection, while the pro-inflammatory cytokines and chemokines that they induce lead to recruitment and activation of antigen presenting cells, B cells, and T cells. The TLRs can modulate the nature of the adaptive immune-responses to give appropriate protection via dendritic cell-activation and cytokine release (Akira S. et al, Nat. Immunol., 2001: 2, 675-680). The profile of the response seen from different TLR agonists depends on the cell type activated.
TLR7 is a member of the subgroup of TLRs (TLRs 3, 7, 8, and 9), localised in the endosomal compartment of cells which have become specialised to detect non-self nucleic acids. TLR7 plays a key rôle in anti-viral defence via the recognition of ssRNA (Diebold S. S. et al, Science, 2004: 303, 1529-1531; and Lund J. M. et al, PNAS, 2004: 101, 5598-5603). TLR7 has a restricted expression-profile in man and is expressed predominantly by B cells and plasmacytoid dendritic cells (pDC), and to a lesser extent by monocytes. Plasmacytoid DCs are a unique population of lymphoid-derived dendritic cells (0.2-0.8% of Peripheral Blood Mononuclear Cells (PBMCs)) which are the primary type I interferon-producing cells secreting high levels of interferon-alpha (IFNα) and interferon-beta (IFNβ) in response to viral infections (Liu Y-J, Annu. Rev. Immunol., 2005: 23, 275-306).
Administration of a small molecule compound which could stimulate the innate immune response, including the activation of type I interferons and other cytokines via Toll-like receptors, could become an important strategy for the treatment or prevention of human diseases. Small molecule agonists of TLR7 have been described which can induce interferon alpha in animals and in man (Takeda K et al, Annu. Rev. Immunol., 2003: 21, 335-76). TLR7 agonists include imidazoquinoline compounds such as imiquimod and resiquimod, oxoadenine analogues and also nucleoside analogues such as loxoribine and 7-thia-8-oxoguanosine which have long been known to induce interferon alpha (Czarniecki. M., J. Med, Chem., 2008: 51, 6621-6626; Hedayat M. et al, Medicinal Research Reviews, 2012: 32, 294-325). This type of immunomodulatory strategy has the potential to identify compounds which may be useful in the treatment of allergic diseases (Moisan J. et al, Am. J. Physiol. Lung Cell Mol. Physiol., 2006: 290, L987-995), viral infections (Horcroft N. J. et al, J. Antimicrob. Chemther, 2012: 67, 789-801), cancer (Krieg A., Curr. Oncol. Rep., 2004: 6(2), 88-95), other inflammatory conditions such as irritable bowel disease (Rakoff-Nahoum S., Cell., 2004, 23, 118(2): 229-41), and as vaccine adjuvants (Persing et al. Trends Microbiol. 2002: 10(10 Suppl), 532-7).
More specifically, allergic diseases are associated with a Th2-biased immune-response to allergens. Th2 responses are associated with raised levels of IgE, which, via its effects on mast cells, promotes a hypersensitivity to allergens, resulting in the symptoms seen, for example, in asthma and allergic rhinitis. In healthy individuals the immune-response to allergens is more balanced with a mixed Th2/Th1 and regulatory T cell response. TLR7 ligands have been shown to reduce Th2 cytokine and enhance Th1 cytokine release in vitro and to ameliorate Th2-type inflammatory responses in allergic lung models in vivo (Duechs M. J., Pulmonary Pharmacology & Therapeutics, 2011: 24, 203-214; Fili L. et al, J. All. Clin. Immunol., 2006: 118, 511-517; Tao et al, Chin. Med. J., 2006: 119, 640-648; Van L. P. Eur. J. Immunol., 2011: 41, 1992-1999). Thus TLR7 ligands have the potential to rebalance the immune-response seen in allergic individuals and lead to disease modification. Recent clinical studies with the TLR7 agonist have shown repeated intranasal stimulation of TLR7 to produce a sustained reduction in the responsiveness to allergen in patients with both allergic rhinitis and allergic asthma (Greiff L. Respiratory Research, 2012: 13, 53; Leaker B. R. et al, Am. J. Respir. Crit. Care Med., 2012: 185, A4184).
In the search for novel small molecule inducers of human interferon IFNα an assay strategy has been developed to characterise small molecule (regardless of mechanism) which is based on stimulation of primary human donor cells or whole blood with compounds, and is disclosed herein.